(1) Field of the Invention
This disclosure relates to a thermoelectric conversion material utilized for thermoelectric power generation and thermoelectric cooling.
(2) Description of Related Art
The thermoelectric power generation is a technology of converting thermal energy directly into electric energy by utilizing the Seebeck effect i.e., thermoelectromotive force generated between both ends of a substance by a temperature difference made between the both ends of the substance in proportion to the temperature difference. This technology is practically used as a power source for a remote place, a power source for space use, a power source for military use, etc. in some cases. The thermoelectric cooling is a technology using the Peltier effect, i.e., a phenomenon of transferring heat through electrons carried by an electric current. Specifically, the thermoelectric cooling is a technology of absorbing heat of a joint part by utilizing a fact that when an electric current is applied to two substances different in polarity of electric conduction carriers connected thermally in parallel and electrically in series, the difference in polarity of the electric conduction carriers (carriers) is reflected on a difference in direction of a heat flow. For example, the two substances different in polarity of electric conduction carriers used in this case are a p-type semiconductor having the electric conduction carriers (carrier) that are holes and an n-type semiconductor having the electric conduction carriers (carriers) that are electrons. Such an element configuration is of a so-called π type and is a most common configuration.
The energy conversion efficiency between heat and electricity in thermoelectric power generation and thermoelectric cooling is determined by a figure of merit ZT of material used. The figure of merit ZT is expressed by using a Seebeck coefficient S, an electric resistivity ρ, and a thermal conductivity κ of the material and an absolute temperature T of evaluation environment as ZT=S2T/ρκ. The energy conversion efficiency becomes higher when the figure of merit ZT is higher, and the high figure of merit ZT is often achieved in a semiconductor into which electric conduction carriers are injected. Therefore, a semiconductor with a low thermal conductivity κ is an important condition for achieving the high figure of merit ZT.
Additionally, a power factor PF=S2/ρ is often used as an index of material performance in thermoelectric power generation. The power factor PF is proportional to output power acquired in thermoelectric conversion and, therefore, large electric energy per unit time can be acquired by using a thermoelectric conversion material with a high power factor PF.
X3Ni3Sb4 (X=Zr, Hf) is one of the semiconductors with a low thermal conductivity κ and is proposed as a candidate for the thermoelectric material. The synthesis of this compound is already attempted and it is described that the thermal conductivity κ at room temperature is 4.3 W/(m·K) in the case of X=Zr and is 2.7 W/(m·K) in the case of X=Hf (see, e.g., Nonpatent Literature 1).
Nonpatent Literature 1:
    J. R. Salvador, X. Shi, J. Yang and H. Wang, “Synthesis and transport properties of M3Ni3Sb4 (M=Zr and Hf): An intermetallic semiconductor”, Physical Review B 77, 235217, Jun. 27, 2008